In recent years, as the processing speed and integration of semiconductor chips have become higher, there has been a growing tendency to replace aluminum or aluminum alloy as a metallic material for forming interconnection circuits on a semiconductor substrate with copper having a low electric resistivity and a high electromigration resistance. These kinds of copper interconnections are generally formed by filling copper into fine recesses formed in the surface of the substrate. As a method for forming copper interconnections, a plating process has generally been performed. When copper interconnections are formed on a substrate by an electrolytic plating process, a seed layer, which is used as a feeding layer in the electrolytic plating, is formed on the surface of a barrier layer made of TaN or the like, and then a copper film is formed on the surface of the seed layer by the electrolytic plating process. Thereafter, the surface of the substrate is polished to a flat finish by a chemical mechanical polishing (CMP) process.
In the case of interconnections formed by such a process, the embedded interconnections have exposed surfaces after the planarizing process. When an additional embedded interconnection structure is formed on such exposed surfaces of the interconnections, the following problems may arise. For example, during formation of an interlayer dielectric of SiO2 in the next process, the exposed surfaces of the pre-formed interconnections are likely to be oxidized. Further, upon etching of the SiO2 layer to form contact holes, the exposed surface of the pre-formed interconnections may be contaminated at the bottoms of the contact holes by an etchant, a peeled resist, and the like. Moreover, in the case of copper interconnections, it is feared that copper may be diffused.
In view of the above drawbacks, for example, it may be considered to selectively cover the surface of the copper interconnections with a protective layer (plated film) of Ni—B alloy or the like which has a good adhesion to a metallic material for forming interconnection circuits, such as silver or copper, and has a low resistivity (ρ). The Ni—B alloy film can be formed selectively on the surface of the copper film or the like by an electroless plating process with the use of an electroless plating solution that contains nickel ions, a complexing agent for nickel ions, and an alkylamine borane or a borohydride compound as a reducing agent for nickel ions, for example.
When a seed layer is formed on the surface of a substrate by an electroless plating process, or when a protective layer (cap plated film) is formed on the surface of a copper film or the like by an electroless plating process, it is necessary to perform pretreatment processes including an Sn adsorption process for substituting Sn for Pd with an SnCl2 solution or the like and a Pd substitution process for depositing Pd as a catalyst in the electroless plating process on the substrate with a PdCl2 solution or the like. Additionally, these processes need a rinsing process (cleaning process) with pure water, respectively. The two-stage pretreatment process may be performed with the use of a catalyst of one solution of Pd/Sn colloid. In this case, it is also necessary to perform a rinsing process with pure water.
The pretreatment process of the electroless plating process (Sn adsorption process or Pd substitution process) and the rinsing process after the pretreatment process (cleaning process) have heretofore been performed as follows. A substrate is laterally held by a vertically movable substrate holder and immersed (dipped) into a treatment solution in a treatment chamber. Alternatively, a substrate is held by a substrate holder so that the surface to be processed faces upward, and a treatment solution is supplied to the upper surface (to be processed) of the substrate held by the substrate holder.
The pretreatment process of the electroless plating process in which the substrate is immersed in the treatment solution utilizes a diffusion effect, and the solution is not positively supplied to the surface, to be processed, of the substrate. Therefore, contaminants attached to the surface to be processed and dissolved ions are removed from the surface of the substrate so slowly that the processing time becomes long, and the surface of the substrate may be recontaminated by the contaminants and the dissolved ions.
In the pretreatment process of the electroless plating process in which the substrate is held so as to face upward, it is necessary to seal the peripheral portion of the substrate by a sealing member in order to prevent the reverse side of the substrate from being contaminated. When the peripheral portion of the substrate is thus sealed by the sealing member, the treatment solution is collected on the surface of the substrate. In this manner, this method also has the same problems as the immersing process described above. On the other hand, if the sealing member is not used, then the reverse side of the substrate cannot effectively be prevented from being contaminated.